An online whiteboard is an application which enables multiple users to share information by the use of objects such as text and figures via a network. Using an online whiteboard, multiple users can synchronously add or modify a text object for inputting text to a whiteboard or a figure object for drawing a figure on a whiteboard, from multiple terminals. In addition, an online whiteboard can provide multiple whiteboards for activities such as discussions and work operations. Hence, it is conceivable for a whiteboard to deal with various situations such as one in which multiple users have a discussion about a common theme and one in which multiple users make reviews of or make comments on the whiteboard at different time points.
Regarding a whiteboard, there is a tendency for a single or multiple users to add or update an object for a discussion or a work operation within a particular time period. Users have a desire to view the state of the whiteboard corresponding to a certain previous time point and a desire to check, on the basis of difference between particular time periods to be displayed, the objects added or modified between the particular time periods. To fulfill such user desires, sharable online whiteboards generally have the function of showing the state of a whiteboard corresponding to a certain previous time point and the function of displaying difference between particular time periods.
Japanese Patent Application Publication No. 2004-336289 discloses a whiteboard record reconstructing method for reconstructing operations performed on a whiteboard in a shared-whiteboard system including a server and a client connected to each other via a network.
In the method of Japanese Patent Application Publication No. 2004-336289, to store information on an input on a whiteboard, the client or the server stores, as a file, a snapshot of the information on the input on the whiteboard. The corresponding one of the client and the server stores following operations performed on the whiteboard, as records, in the file together with time information in accordance with a certain rule. Through the use of the method of Japanese Patent Application Publication No. 2004-336289, it is possible to reconstruct operations performed on a whiteboard on the basis of a stored snapshot and stored records.